Microstructure-capacitive performance relationship of carbon cathodes for zinc ion hybrid capacitors: Effect of edge nitrogen/surface area and micropore/mesopore ratios

Abstract

Clarifying microstructure-performance relationship is significant to rationally design high-performance porous carbon cathodes for zinc ion hybrid capacitors (ZIHCs). However, the microstructure of porous carbons evolves irregularly due to the complex preparation process. Herein, a cyanamide mediating potassium carbonate (K₂CO₃) activation strategy is developed to orderly regulate the micropore/mesopore and edge nitrogen/surface area ratios of nitrogen-doped lignin-derived porous carbons (NLPCs). Cyanamide units react with K₂CO₃ to generate KOCN and edge-nitrogen framework below 500 °C. As temperature increases from 500 to 900 °C, KOCN gradually reacts with carbon to increase surface area and generate KCN template to construct mesopore, negatively-gradient regulating the micropore/mesopore ratio from 14.0 to 0.6. Simultaneously, the edge-nitrogen framework is integrated into carbon skeleton to construct edge nitrogen, and gradually depleted by reacting with K₂CO₃ to generate KOCN, negatively-gradient regulating the edge nitrogen/surface area ratio from 3.6 to 0.3 at.% m⁻² mg. The micropores and edge nitrogen can be maximally utilized for Zn²⁺ ion storage under micropore/mesopore ratio of 0.6 and edge nitrogen/surface area ratio of 0.6 at.% m⁻² mg, achieving an excellent capacitance of 357 F g⁻¹ and robust rate capability. The assembled ZIHCs display the highest energy density of 126 Wh kg⁻¹ at 80 W kg⁻¹.